ICU analysis and backwards compatibility

From time to time, the ICU library receives updates such as adding new
characters and emojis, and improving collation (sort) orders. These changes
may or may not affect search and sort orders, depending on which characters
sets you are using.

While we restrict ICU upgrades to major versions, you may find that an index
created in the previous major version will need to be reindexed in order to
return correct (and correctly ordered) results, and to take advantage of new
characters.